Skin cancer, in the form of melanoma, basal or squamous cell carcinoma, is the most commonformof cancer in the U.S.A. and is mainly caused by sun exposure, which results in DNA damage and photocarcinogenesis. There is mounting evidence for the significance of the melanocortin 1 receptor (MC1R) that is expressed on human melanocytes (hMC) and its endogenous ligands a-melanocyte stimulating hormone (a-melanocortin;a-MSH) and adrenocorticotropic hormone (ACTH) in photoprotection against skin cancer. First, activation of the MC1R by its ligands increases the synthesis of the photoprotective eumelanin, the black-brown form of melanin. Second, loss-of-function mutations in the human MC1R gene are associated with red hair phenotype, poor tanning ability and increased risk for skin cancer. Certain mutations in the gene for proopiomelanocortin, the precursorfor melanocortins, also result in red hair phenotype. Third, we discovered a novel role for a-MSH as a survival factor that rescueshMC from ultraviolet radiation (UVR>induced apoptosis and reduces DNAdamage. These effects are absent in hMC expressing loss-of-function MC1R alleles, which exhibit a reduced DNA repair capacity. Based on this evidence, the main goal of this proposal is to develop a new skin cancer preventative strategy based on utilizing potent synthetic agonists of a-MSH that can be delivered topically. Our hypothesis states that synthetic a-MSH agonists augment photoprotection in human skin and prevent skin cancer by recapitulating the stimulatory effects of a-MSH on melanogenesis, as well as on the survival and reduction in DNA damage of hMC. To investigate this hypothesis, three specific aims are proposed. The goal of Specific Aims 1and 2 is to design and synthesize potent, stable, long acting fragment analogs of a- MSH and test their ability to recapitulate all the effects of a-MSH on hMC by selectively binding and activating the MC1 R. In Specific Aim 3, the goal is to test the effects of the most effective agonists on cultured skin substitutes containing normal hMC, as well as hMCfrom individuals with a high risk for skin cancer (carriers of mutations in the MC1R or p16INK4Agene), and evaluate the possible toxicological effects and percutaneous permeability of these agonists. The significance of our proposed skin cancer prevention strategy lies in utilizing potent synthetic fragmentanalogs of a-MSH that are selective super agonists for the human MC1R and augment the photoprotection of the skin by reducing UVR-induced DNA damage and increasing eumelanin synthesis. This strategy will ultimately reduce the incidence of skin cancer particularly in high-risk population, such as individuals heterozygous for a loss-of-function MC1R allele or expressing mutations in other skin cancer susceptibility genes, such as the melanoma susceptibility gene p16INK4A.